It has been shown in S. Mladenovski et al., “OLED light outcoupling enhancement by Nelder-Mead layer thickness optimization”, 2008 International Conference on the Science and Technology of Emissive Displays and Lighting, EL 2008 Proceedings, pp. 9-12, that for single color OLED devices the light output into air can be about doubled at e.g. the green wavelength region by applying a tuned so called double resonant cavity structure (RC2), i.e. a sequence of high and low index layers with optimized thicknesses between one of the electrodes of the OLED and the substrate. Usually two extra high index layers and one low index layer in between are sufficient to form the additional microcavity.
In order to provide a white light emitting device based on OLEDs, different layers emitting light with different colors, in particular red, green and blue light, are normally combined in one layer stack. Through the combination of the different colors, the device emits the desired white light. In order to improve the luminous power efficacy of such a white light OLED, the above technique of the double resonant cavity structure can not be applied since the tuning of the additional microcavity only improves the efficacy of one of the colors in a range of about 90 nm, wherein outside of this wavelength range the light emission is strongly reduced. Applying this concept for such a white light OLED would thus improve the efficiency only for one color and reduce the radiance of the other colors.